Two-way telephone repeater



May 24, 1938. 1.. w. BRANCH TWO-WAY TELEPHONE REPEATER Filed April 18, 1936 ET ETL T INVENTOR ZesZzk Mfimnek,

Y E N R O T T A WITNESS Patented May 24, 1938 UNITED STATES PATENT OFFICE.

5 Claims.

This invention relates to telephone repeaters, and has for an object to provide a simplified two direction amplifying repeater for long distance telephone transmission.

Ordinarily, telephone repeaters use the Wheatstone bridge method of balancing the current in one arm of a circuit against that in another arm to prevent energy from doubling back in an amplified state to the transmitting station. An artificial line is used to balance against the actual, or real line used for the transmission of speech. This artificial line, to be effective, must have the same electrical characteristics, inductance, resistance, capacity, and so forth, as the real line. It is well known that it is practically impossible to maintain an open telephone line of constant characteristics. Such lines change more or less with every change of weather and any foreign object coming in contact with the line will also change its characteristics. The characteristics of the artificial lines, which are usually indoors, remain practically constant un-- less varied by an attendant. This makes it necessary to have expert attendants at practically all repeater stations to keep the artificial and real lines in balance.

Furthermore, artificial lines associated with ordinary repeaters absorb about fifty percent of the energy arriving at a repeater station. This energy is lost completely. A high degree of amplification is therefore necessary to overcome not only the losses due to line attenuation but the fifty percent loss in the artificial line as well. High amplification has objectionable features, principally distortion, which in many cases be come so great as to make speech unintelligible.

The primary object of the present invention, therefore, is to overcome all of the above mentioned undesirable features by the elimination of an artificial line with its attendant energy ab-. sorption which must be made up by amplification. My invention uses a moderate degree of amplification just sufficient to overcome losses due to line attenuation and coil losses, which latter are small.

With the above and other objects in View the invention consists of certain novel details of construction and combinations of parts hereinafter fully described and claimed, it being understood that various modifications may be resorted to within the scope of the appended claims without departing from the spirit or sacrificing any of the advantages of the invention.

In the accompanying drawing forming part of this specification,

The figure is a diagrammatic view showing the application of the invention to a two-way one repeater circuit.

Referring now to the drawing in which like characters of reference designate similar parts in the various views, transformers 9 and III are shown each having a laminated core formed of sheets of Permaloy metal cut and arranged to form parallel longitudinal legs I I and I2 connectby three transverse legs I3, I4, and I5, the transformers having in general the contour of the figure 8. The two transformers are identical in all respects, with each other. The transformer 9 is associated with the telephone line I from a distant telephone station and the transformer Iii is associated with a telephone line 2, to a distent telephone station. On the central leg of each transformer is wound a line coil I6 having approximately 2,000 turns of Wire and a direct current resistance of approximately 50 ohms, the coil being connected in series with the associated telephone line. The center leg is of greater diameter than the end legs to short circuit and prevent a magnetic field set up in one end leg from passing to the other end leg. 2

On one outer leg I3 of each transformer there is wound a grid coil I! having approximately 10,000 turns of wire and a direct current resistance of approximately 5,000 ohms. There are two neutralizing coils I8 wound on the same leg of the transformer outside of the grid coil and having approximately 2,000 turns of wire and a direct current resistance of approximately 750 ohms.

A plate coil I9 is wound on the other outer leg 31- iii of each transformer, the plate coil having approximately 5,000 turns of wire and a direct current resistance of about 2,500 ohms.

The repeater also includes a pair of three element vacuum tubes and El, the former being associated with the incoming telephone line I and its transformer and the latter being associated with the outgoing telephone line 2 and its transformer. The circuit diagram shows only two amplifying tubes for each repeater and this is sufficient to meet average conditions. If greater amplification is required, additional tubes may be used.

2i! indicates the A battery, 2| the B battery, and 22 the C battery, 23 the choke coils, and 24 condensers associated with the amplifying tubes.

Alternating current of voice frequencies, sent from distant telephone station over the telephone line I, due to line attenuation, will be weak, while passing through the line coil I6 of the transformer 9, but it will be of sufficient strength to set up an alternating magnetic flux in the core of the transformer 9. This alternating magnetic field induces in the grid coil H of the transformer alternating current identical in wave shape to that passing in the line coil l6.

No current will be induced in the plate coil l9, and in the neutralizing coils l8 on the core of the transformer 9, as the circuit to which they are connected is open in the tube 2|, one conductor wire 25 of this circuit being connected to the plate 26 of the tube 2|, while the other conductor wire 21 of this circuit is connected to the filament 2B of the same tube. As electron tubes will only operate in one direction the tube 2! has the effect of opening the circuit of the plate coil I9 and neutralizing coils l8 of the transformer 9 associated with the incoming telephone line I.

The grid coil ll of the transformer 9 is connected to the grid 29 of the vacuum tube 20 by a conductor wire 30 and connected to the filament 35 of this tube by a wire 32 connected to the negative terminal of the C battery 22. The plate 33 of the tube 20 is connected by a wire 34 to the plate coil l9 of the transformer H] which is associated with the outgoing telephone line 2. The varying voice impulses acting on the grid 29 will cause a greatly amplified current to pass from the B battery 2| through the tube 2|] and plate coil l9 associated with the outgoing telephone line 2. This amplified current will induce a strong magnetic flux in the core of the transformer ID. This varying magnetic field will induce a strong alternating current of voice frequencies in the line coil l6 of. the transformer I 0 associated with the outgoing telephone lines and thence to the distant telephone station. Thus the weak current arriving at the repeater of the incoming tele phone line I is greatly amplified and transmitted over the outgoing telephone line 2. Weak currents arriving at the repeater of the telephone line 2 will follow the same course as above described except that they will be amplified in tube 2! instead of in tube 20, since the grid coil of the transformer I0 is connected to the grid 35 of the tube 2|, by a wire 36 and t0 the filament 28 of this tube by a wire 3! connected to the negative terminal of the C battery in the same manner as above described in connection with tube 20.

It might appear that when the amplified current from the tube 20 is passed through the plate coil and neutralizing coils of the transformer l0 associated with the telephone line 2, current would be induced in the grid coil as well as in the line coil on this transformer. This, however, is not the case. There is inductive action between the plate and line coils, but not between the plate and grid coils. This is an essential characteristic of my repeater and makes its use in two directions with one pair of wires possible.

The action of the five coils wound on the core of the transformer is as follows.

When an alternating current is flowing in the plate coil l9 the magnetic flux, when kept at ordinary densities, such as are produced by voice currents, produced by this coil passes through its core and the core of the line coil, the latter as above described being wound on the central leg of the transformer. This central leg acts as a magnetic short circuit and no magnetic lines pass through the core of the grid coil on the end leg. I have found by experiment that electrical energy may be inductively transferred from the plate to the line coil without any effect whatever being manifested in the grid coil. However, when the induced current begins to flow in the line coil it sets up a magnetic flux of its own which is opposed to that set up by the plate coil. As it is impossible to build a perfect transformer the magnetic fields of these two coils can never be made to actually balance each other.

Therefore, when current is flowing in both the plate and line coils at the same time, the line coil being fed by the plate coll, a weak resultant magnetic flux produced by the combined action of the two coils passes through the core of the grid coil and produces a very weak current relative to that flowing in the line coil, in the grid coil. This, of course, would actuate the amplifying tube 21 and cause the voice currents intended to be transmitted to telephone line 2 to double back to telephone line I resulting in serious reflections which would greatly hamper, if not completely destroy telephonic transmission. To neutralize this inductive effect in the grid coil I provide the two neutralizing coils 18 of small dimensions relative to the plate and line coils, located on opposite sides of. the grid coil. These neutralizing coils are connected in series with the plate coil by a conductor wire as shown 33, and, by reason of their location on the core the magnetic effect produced by these neutralizing coils neutralizes the magnetic effect produced by the resultant action of the plate coil and the line coils. In other words, the neutralizing coils set up a counter-flux opposing the flux tending to induce an electro-motive force in the grid coils.

The above described arrangement completely eliminates the current induced in the grid coil, when the line coil is being fed from the plate coil, and accomplishes this without adversely effecting the inductive action between the plate and line coils. In practice, I have found that by using t .e neutralizing coils the transfer of energy from the plate coil to the line coil is augmented instead of retarded which is a desirable advantage.

From the adove description it will be seen that voice frequency energy arriving over telephone line I will be transferred from the line to the grid coil of the transformer 9 associated with line I. The energy induced in this grid coil will actuate the tube 20 associated with the transformer 9 and in this tube the energy will be greatly amplified and fed through the plate coil and neutralizing coils associated with the transformer It in the outgoing telephone line 2, then transferred to the line coil of the transformer and thence over the outgoing telephone line 2. Enengy arriving at the repeater over the line 2 follows the same course through the amplifying tube 2! to the transformer 9 and thence to telephone line I.

As before stated, electron tubes will pass energy in one direction only, that is, from the filament to the plate. Consequently, in a repeater two tubes at least must be used, one to pass energy in one direction and one to pass energy in the opposite direction, and the circuit wiring must be such that when one tube is passing energy in one direction, the tube intended to pass energy in the opposite direction will not be affected. This essential requirement is embodied in the five coils on the transformer of figure 8 contour. When energy is being transmitted from telephone line I to telephone line 2, tube 20 passes energy but tube 2| is not affected, there being no energy induced in the grid coil associated with the transformer l0 associated with telephone line 2.

When energy is being transmitted from telephone line 2 to telephone line I, the amplifying tube 21 passes energy but the tube 20 is not affected because there is no energy induced in the grid coil of the transformer 9 associated with telephone line I.

From the above description it is thought that the construction and operation of the invention will be fully understood without further explanation.

What is claimed is:

1. A two-way telephone repeater including a pair of transformers each comprising a core having parallel longitudinal legs and three transverse legs connecting the longitudinal legs, a telephone line coil on the center leg of each transformer, a plate coil on one end leg of each transformer, a grid coil on the other end leg of each transformer, a pair of amplifier tubes, a grid and plate circuit for each tube including respectively the grid coil of one transformer and the plate coil of the other, and neutralizing coils on the same legs of the transformers as the grid coils connected in series with the plate coils of their respective transformers and adapted to set up a counter-flux opposing the flux tending to induce an electro-motive force in the grid coil.

2. A two-Way telephone repeater including a pair of transformers each comprising a core having parallel longitudinal legs and three transverse legs connecting the longitudinal legs, a telephone line coil on the center leg of each transformer, a plate coil on one end leg of each transformer, a grid coil on the other end leg of each transformer, a pair'of amplifier tubes, a grid and plate circuit for each tube including respectively the grid coil of one transformer and the plate coil of the other, and neutralizing coils on the same legs of the transformers as the grid coils connected in series With the plate coils of their respective transformers and adapted to set up a counter-flux opposing the flux tending to induce an electro-motive force in the grid coil, the center leg of each transformer being of materially greater cross section than the end leg to shunt the magnetic flux due to the plate coil from the leg carrying the grid coil.

3. A two-way telephone repeater including a pair of transformers each comprising a core having parallel longitudinal legs and three transverse legs connecting the longitudinal legs, a telephone line coil on the center leg of each transformer, a

plate coil on one end leg of each transformer, a grid coil on the other end leg of each transformer, a pair of amplifier tubes, a grid and plate circuit for each tube including respectively the grid coil of one transformer and the plate coil of the other, and neutralizing coils on the same legs of the transformers as the grid coils connected in series with the plate coils of their respective transformers and adapted to set up a counter-flux opposing the flux tending to induce an electro-motive force in the grid coil, the neutralizing coils of each transformer being disposed at opposite ends of the grid coil.

4. A two-way telephone repeater including a pair of transformers each comprising a core having parallel longitudinal legs and three transverse legs connecting the longitudinal legs, a telephone line coil on the center leg of each transformer, a plate coil on one end leg of each transformer, a

grid coil on the other end leg of each transformer,

a pair of amplifier tubes, a grid and plate circuit for each tube including respectively the grid coil of one transformer and the plate coil of the other, and neutralizing coils on the same legs of the transformers as the grid coils connected in series with the plate coils of their respective transformers and adapted to set up a counter-flux opposing the flux tending to induce an electro-motive force in the grid coil, the center leg of each transformer being of materially greater cross section than the remaining legs to shunt from the grid coil carrying legs the magnetic flux due to the plate coils, the neutralizing coils being disposed at the opposite ends of the grid coils.

5. A two-way telephone repeater including a pair of transformers each comprising a closed core and an intermediate leg spanning the core, a telephone line coil Wound on said intermediate leg, grid and plate coils wound respectively on remote ends of the core on opposite sides of the intermediate leg, a pair of amplifying tubes, a grid and plate circuit for each tube including respectively the grid coil of one transformer and the plate coil of the other transformer, and neutralizing coils wound on the core at opposite ends of the grid coil, the neutralizing coils of each transformer being energized from the plate circuit in which the plate coil of that transformer is included.

' LESLIE W. BRANCH. 

